1. Field of the Invention
This invention relates generally to a solid state image sensor, and is directed more particularly to a charge coupled image sensor of the inter-line transfer type.
2. Description of the Prior Art
In the art it is known to employ a solid state image sensor comprising, for example, a charge coupled device or element (CCD) arranged as an interline transfer system.
FIG. 1 shows a typical charge coupled device 10 of the prior art, which consists of a common semiconductor substrate 1. A number of sensing areas 2 are formed on the semiconductor substrate 1 and aligned as picture elements covering a two-dimensional matrix or area. Vertical shift register 3 is provided adjacent each vertical column of sensing areas 2 in the horizontal scanning direction. Each of the vertical shift registers extend in a vertical direction. A horizontal shift register 4 is provided for transferring the stored carriers to an output terminal.
In order to obtain an output signal from the solid state image sensor 10, minority carriers stored in each sensing area 2 in response to light information are transferred during the vertical blanking interval of a television signal at every vertical line to the vertical shift register 3 (parallel transfer). Secondly, the carriers are transferred through each vertical shift register 3 in the vertical direction (serial transfer) sequentially. Finally, the carriers are read out through the horizontal shift register 4 at every horizontal scanning line. Thus, a desired output signal can be obtained at the output terminal (not shown) of the horizontal shift register 4.
Since the transfer of stored carriers is carried out at every horizontal scanning line, this transfer system is generally called an interline transfer (or interline) shift system.
The arrows appearing in FIG. 1 show the direction of the carrier transfer. When an interlace scanning system is employed during image picking up, the dotted line arrows in FIG. 1 show carrier transfers in the even fields. Thus, the stored carriers are transferred to the vertical shift registers 3 only through the paths shown by solid line arrows in the odd fields.
To obtain sufficient resolution with the above prior art solid stage image sensor 10, it becomes necessary to increase the number of picture elements and hence it becomes very difficult to form a solid state image sensor on the semiconductor substrate 1 of predetermined dimensions without serious flaws developing.
Furthermore, the carrier transfer efficiency of the vertical shift registers of the prior art solid state image sensor is poor. Therefore, it has been difficult to form a practical solid state image sensor.